In fact, why was the Soviet Union so interested in Mongolia during World War II?

The Soviet Union is not only interested in Mongolia, but also in carving up the whole of China. During tsarist Russia, he broke away from Mongolia and became his puppet. It was President Roosevelt's decision to send troops to the northeast of China. In order to relieve the pressure of American troops, Roosevelt asked the Soviet Union to declare war on Japan within three months after the end of the European War. However, 44 years later, Japanese resistance no longer poses a threat to American troops. Roosevelt regretted it, but the Soviet Union didn't agree, so he planned to defeat Japan before the Soviet Union captured Berlin, but the Soviet Union invaded Berlin in advance. So he planned to end the war before the Soviet troops were transferred from the western front to the Far East, but the abacus failed. The Soviet Union entered the war in Japan in time and shared the fruits of the victory of World War II in Asia. If the Soviet Union had not participated in the Asian battlefield, Japan's four northern islands and South Korea would not have become the sphere of influence of the Soviet Union.